Isomerization is often used to increase the octane rating of light hydrocarbons, such as pentane, hexane, and heptane used in gasoline. For example, a single pass of some hydrocarbons, such as pentane and hexane mixtures, through an isomerization unit can increase the octane rating from about 50-60 to about 80-93. Butanes are not typically used in gasoline, but butanes are isomerized to produce more valuable products as well. The catalyst used in many isomerization units is in a chlorided state, and oxygenates in the feed produce water which displaces chlorine on the catalyst. The displaced chlorine is discharged as hydrochloric acid (HCl), and a chloriding agent is added to the isomerization process to replace discharged chlorine. Water acts as a poison to the catalyst and reduces the catalyst life expectancy. Because one of the primary oxygenates in many feed streams is water, the feed stocks are dried to remove as much water as possible before the reaction. Any other oxygenates in the feed generally react and form water.
The isomerized product is fractionated, and the hydrochloric acid exits the fractionation column with excess hydrogen and other low boiling compounds typically in an overheads stream. The hydrochloric acid in the overheads stream is corrosive, which further complicates treatment or other processing needed to re-use the discharged hydrogen. The overheads stream is often treated in a sodium hydroxide (caustic) scrubber to remove the hydrochloric acid, and then is disposed of or further processed. Hydrogen is a valuable compound in the off gas from the caustic scrubber, but water in the off gas discharged from the caustic scrubber makes re-use in the isomerization unit impractical.
Make-up hydrogen is added to the isomerization reactor, and dual molecular sieve dryers are often used to remove any entrained water. The molecular sieves typically used for drying the hydrogen and hydrocarbon feed streams need to be regenerated frequently and is usually replaced about every 2 to 4 years, which increases maintenance costs and capital costs to provide duplicate equipment to run the isomerization unit when one molecular sieve dryer is out of service for the regeneration or replacement of the adsorbent.
Accordingly, it is desirable to develop methods and apparatuses for drying and recycling hydrogen discharged from hydrocarbon isomerization units, including recycling of discharged chloriding agents such as HCl. In addition, it is desirable to develop methods and apparatuses for drying hydrogen make-up feed stocks with lower maintenance and operating costs. Furthermore, other desirable features and characteristics of the present embodiment will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and this background.